Earth came together like a fluid continental puzzle

By now almost everyone knows that the continents of today were not always as they are: 250 million years ago they were all connected in the supercontinent Pangaea.

What many people don’t realize is that before Pangaea, the continents were separate. Before that, they were together in a previous supercontinent called Rodinia; before that they were separate, and before that they were together in

a previous supercontinent called Columbia (by some geologists) or Nuna (by others). And the pieces were not always the same — land that was part of one continent at one time might have been part of a different continent another time.

Geologists have a very good handle on how the pieces of Pangaea were arranged, but the further back in time you go, the harder it is to put the pieces back together. There is some disagreement about how Rodinia was assembled, and a lot of disagreement about Columbia.

Recent research by an international team of scientists from Australia, France and the United States had some fascinating information on two pieces of Rodinia. That supercontinent came together just over a billion years ago, stayed together for about 300 million years, and broke apart around 700 million years ago.

The researchers discovered that the area containing our Grand Canyon in Arizona was once contiguous with land that is now part of Tasmania, the island state of Australia that’s about 150 miles off its southern coast.

Deep in the Grand Canyon are some rocks called the Unkar Group. They formed 1.25 to 1.10 billion years ago and include shales and siltstones overlaid by dolostone (a kind of limestone) containing stromatolites (colonies of blue-green bacteria). That is overlaid by a layer of sandstone containing soft-sediment deformation features that grade upward into finergrained siltstone.

Rocks of the Rocky Cape Group in Tasmania have the same kinds of rocks, in the same sequence, deposited at the same time. That in itself would be suggestive of a connection, but the geologists offered more evidence.

The mineral zircon is tough. Once its crystals form, usually in granite type rock, those crystals can be eroded and deposited multiple times without their initial time of formation being changed. The sandstones of both the Unkar Group in Arizona and the Rocky Canyon Group in Tasmania contain populations of zircon crystals with similar ages, suggesting their source areas were the same.

Also, when an ancient lava flow erupts, the tiny crystals of iron-bearing minerals line up like compass needles. When the lava cools and hardens, that orientation is frozen in the rock. In the new arrangement of the two pieces of Rodinia those ancient compass directions match.

Confirmation could come from an ancient plate boundary that might be hidden under the ice of East Antarctica.

Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University. gnidovec.1@osu.edu